Hello: I was searching online for M6 X 12 sealing washers. These are used on the Tomos A55 transmission at the fluid level check screw. I discovered a source in France called 123Bearing.com. They had what I needed (part number RDL-6X12X1-FIBRE) currently priced at 46 cents apiece. I placed an order for 10 washers on Friday, September 29, 2017 and was amazed when the package arrived today, Wednesday, October 4th. That's pretty impressive service. They also have a stateside phone number (646-712-9672). Give them a try. Maybe they have what you need as well.

Hello: Thank you for the tip about trying the plumbing suppliers. I had gone that route, but not one of them in this area (Flint, MI) had what I needed. Nonetheless, the fiber washers I received from 123Bearing.com are a little thicker with slightly tighter tolerances than the original copper washer. And, so far there's no sign of any seepage. I'll probably go for a ride later tonight 'cause rain is predicted for the next few days.